The present invention relates to polyurethane elastomers and to a process for the production thereof. More particularly, the present invention relates to polyurethane elastomers having improved physical properties, the polyol component and isocyanate-terminated prepolymer or quasi-prepolymer used to produce such elastomers, and to a one-shot process for producing polyurethane elastomers from these materials. Preferably, these elastomers are prepared by chain extending an isocyanate-terminated prepolymer or quasi-prepolymer prepared from a polyol component having a number average molecular weight of from about 1000 to about 3000 Da. This polyol component includes a low molecular weight polyol having a high polydispersity index and a low monol polypropylene glycol.
Polyurethane elastomers are widely used in such diverse applications as gasketing and sealing materials, medical devices, ski boots, jounce bumpers, and conveyor rollers, to name a few. Due to their strength, hardness, and other properties, elastomers prepared from isocyanate-terminated prepolymers or quasi-prepolymers incorporating polytetramethylene ether glycol (PTMEG), polycaprolactone and polyester polyols are predominantly used for demanding applications.
PTMEG, polycaprolactone and polyester polyols tend to be high cost starting materials, however. As a result, polyurethane elastomers prepared from these polyol components are also higher priced products.
Polyoxypropylene diols have been suggested as possible substitutes for PTMEG in elastomer prepolymer formulations, however, the properties of the elastomers thus produced are not comparable to those achieved with PTMEG.
The patent literature teaches the benefits of using low unsaturation polyoxypropylene diols but also recognizes that production of elastomers with such polyols yields products that exhibit low modulus values, low hardness values, low compression deflection and abrasion resistance and presents processing problems.
One approach which has been taken to improve these physical properties and to reduce or eliminate the processing problems encountered with such low unsaturation polyols is the use of a blend of polyols. U.S. Pat. No. 5,648,447, for example, discloses polyurethane elastomers produced from a prepolymer made with a polyol component containing both PTMEG and from 5 to 35 equivalent percent of a low monol polyoxypropylene polyol which is chain extended with an aliphatic diol or an aromatic amine. It is, however, taught in this patent that if more than 35 equivalent percent of low monol polyoxypropylene diol is used, tensile strength of the elastomer rapidly diminishes and elongation values are worse than those for elastomers made using only low monol polyoxypropylene diol. The economic benefit of using low monol polyoxypropylene diols is not therefore fully achieved due to the requirement that less than 35 equivalent percent of such diol be used if tensile strength and elongation values are to be maintained.
It has also been found that approximately 20% more isocyanate (specifically, MDI) is needed in systems such as those disclosed in U.S. Pat. No. 5,648,447 to achieve the same degree of hardness as that obtained using comparable PTMEG systems. Further, the optimum mechanical properties are achieved only if the chain extension of the prepolymer is sufficiently catalyzed that the effective potlife of the system is approximately 2 minutes or less. Elastomers which require processing times of longer than 2 minutes can not therefore be produced with such systems without sacrificing the mechanical properties of the product elastomer.
Among the known processes used to produce polyurethane elastomers, one-shot processes are considered to be particularly advantageous. U.S. Pat. Nos. 5,668,239 and 5,739,253, for example, each disclose a one-shot process for the production of polyurethane/urea elastomers from isocyanate-terminated prepolymers, polyether polyols and a chain extender.
It would therefore be advantageous to develop an elastomer-forming composition in which a significant amount of the polyol component employed is a low monol polyoxypropylene diol which produces elastomers having hardness, modulus, compression deflection, abrasion resistance and processability comparable to those of elastomers currently produced exclusively with traditional high performance polyols.
It is an object of the present invention to provide a polyol component useful for producing polyurethane elastomers having good processing characteristics and physical properties even when the processing time is longer than 2 minutes which includes a significant amount of low cost, low monol polyoxypropylene polyol.
It is another object of the present invention to provide an NCO-terminated prepolymer or quasi-prepolymer useful for the production of polyurethane elastomers having characteristics comparable to those of elastomers produced solely with high performance polyols such as PTMEG, polycaprolactones and polyesters.
It is a further object of the present invention to provide polyurethane elastomers characterized by good hardness, modulus, elongation, abrasion resistance and compression properties.
It is an additional object of the present invention to provide an economical process for the production of polyurethane elastomers having good mechanical properties in which processing of the elastomer-forming materials does not require unacceptably short reaction times.
These and other objects which will be apparent to those skilled in the art are accomplished by using a polyol component which is a mixture or blend having a number average molecular weight of from about 1000 to about 3000 Daltons. This polyol component must include: (1) a significant amount (i.e., greater than 60% by weight, based on total weight of polyol component) of a low monol polyoxypropylene polyol having a number average molecular weight of from about 2000 to 12,000 Da and a degree of unsaturation less than or equal to 0.02 meq/g and (2) a minor amount (i.e., less than 40% by weight, based on total polyol component) of a low molecular weight polyol having a high polydispersity index (i.e., the polydispersity index is greater than 1.1). This polyol component is reacted with an isocyanate, an isocyanate-terminated prepolymer or an isocyanate-terminated quasi-prepolymer. The elastomers produced in accordance with the present invention are most preferably synthesized by chain extension of an isocyanate-terminated prepolymer or quasi-prepolymer prepared by reacting a stoichiometric excess of one or more di- or polyisocyanates with at least a portion of the polyol component or one of the polyols of the polyol component.